1. Field of the Invention
The present invention relates to a focus detecting apparatus and an optical device, and more particularly, to improvement of a camera having the focus detecting apparatus.
2. Description of the Related Art
Conventionally, it has been well known that an image formed by an imaging lens of an optical device is split into two parts by an image refocusing optical system, and the two parts are reformed on a photoelectric converter element array. Then, there have been proposed a number of focus detecting optical systems for detecting displacement of these two parts, thereby carrying out focus detection. Among these proposals, for example, a technique of detecting focus in a particularly wide screen range is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-141980, Jpn. Pat. No. 3363683, and Jpn. Pat. No. 2886865.
In Jpn. Pat. No. 2001-141980 and Jpn. Pat. No. 3363683 described previously, there is described an optical device for allocating a condenser lens function of a focus detecting system in the vicinity of a sub-mirror to reduce size.
In addition, in Jpn. Pat. No. 2886865 described previously, there is described a technique of attempting to expand a focus detecting region by configuring a condenser lens having different optical axes between a center portion and a peripheral portion.
FIGS. 1A to 1C are views showing layouts of an observation optical system and an image pickup optical system which represent the relationship between the size of such a conventional focus detecting range and the size of a sub-mirror.
Although a digital still single-lens reflex camera and a sub-mirror having a flat face have been illustrated for simplification of explanation, this illustration is basically applicable to a sub-mirror acting as a lens or in another optical device as well.
FIG. 1A is a view showing an example of an optical device 10A having a narrow focus detecting range; FIG. 1B is a view showing an example of an optical device 10B having a middle focus detecting range; and FIG. 1C is a view showing an example of an optical device 10C whose focus detecting range is substantially identical to the viewing field (imaging range) of the optical device.
A light beam from an object (not shown) is reflected by a main mirror 14 via an imaging lens 12, and the reflected light beam is guided to an eyepiece optical system 20 via a focusing glass 16 and a pentaprism 18. In addition, luminous flux having been transmitted by the main mirror 14 is reflected downward in FIGS. 1A to 1C by a foldable sub-mirror 24 mounted at the rear of the main mirror 14. Then, the luminous flux reflected by the sub-mirror 24 is guided to a photoelectric converter element 36 incorporated in a focus detecting apparatus. The luminous flux incident into the focus detecting apparatus is reflected by a mirror 30 via a field stop 26 and a condenser lens 28, and the reflected luminous flux is guided to the photoelectric converter element 36 through a diaphragm 32 and a re-imaging lens 34.
In addition, an infrared filter 40, a low-pass filter 42, and an image pickup device 44 are allocated at the rear of the main mirror 14 and the sub-mirror 24. The main mirror 14 and the sub-mirror 24 are retracted from the optical path during imaging so that luminous flux from an object (not shown) is guided to the image pickup device 44 via the infrared filter 40 and the low-pass filter 42.
Here, in an example of the optical device 10A shown in FIG. 1A, the optical device having a narrow focus detecting range “a”, the device can be configured smoothly without any problem. However, in the optical device 10B shown in FIG. 1B, the optical device having a middle focus detecting range “b”, a restriction on configuration occurs in the reflection direction of the sub-mirror 24 or when the main mirror 14 and the sub-mirror 24 are retracted during imaging.
Further, in the case of the optical device 10C shown in FIG. 1C in which the focus detecting range “c” is substantially identical to the viewing field (imaging range) of the optical device, the position of the main mirror 14 is greatly distant from the primary image focusing face in this single-lens reflex camera.